Multiple channel signal transmission system



M. ARTZT Sept. 9, 1941.

MULTIPLE CHANNEL SIGNAL TRANSMISSION SYSTEM SEQ Filed Feb. 29, 1940 wSMN Patented Sept. 9, 1941 MULTIPLE CHANNEL SIGNALTRANSMISQ SION SYSTEM Maurice Artzt, iladdonfield, N.- J., assignor to Radio Corporation of America,

Delaware 1 a'corporation of Application February 29, 1940, Serial No. 321,562

3 I 18 Claims. (Cl. 178-44) The present invention relates to multiple channel signal transmission systems. Moreparticularly, the present invention relates to signal transmission systems of the type having at least two channels for conveying signals in differing complementary frequency bands covering the overall transmission of signals in a predetermined wide frequency range.

Present systems of the type referred to include dual channel audio frequency amplifiers, for example, in which signals in a low audiofrequency portion of the frequency range of the amplifier are selected and conveyed through one channel, while signals in the remainder and higher frequency portion of the operating range of the said amplifier are selected and conveyed through a second channel, thereby permitting separate output circuits and/r sound-producing means in connection with the channels, and a more uniform signal output over the entire audio frequency operating range of the system.

With multiple channel signal transmission systems of the type referred to, the signal transmission frequency bands are chosen to operate above and below a certain frequency, referred to as the cross-over frequency, with sharp cut-off or signal attenuation in any two adjacent frequency channels at that frequency.

Various circuits and expedients have been employed heretofore to give a sharp cut-off-characteristic to the signal attenuation in each of two adjacent frequency signal transmission channels of a multiple channel signal-conveying system, and it is a primary object of the present invention to provide an improved multiple channel signal-transmission system wherein the cut-off or signal attenuation in each of 'two adjacent frequency channels may be increased in sharpness and improved at the cross-over frequency.

In amplifying and other signal transmission systems of the multiple-channel type, it is desirable and often necessary to maintain the phase shift in each channel substantially zero' In the case of a combining circuit for two channels at the output ends thereof, it is necessary to provide means in the several channels to maintain the substantially Zero phase shift above referred to or to provide a complementary phase shift in the several channels in order that the output from each channel may add correctly in phase with the output from the other channels.

It is a further object of the present invention to provide a multiple channel signal-conveying system wherein at least two channels provide such phase shift that the output from said channels may be added in phase, while at the same time maintaining a relatively sharpcut-ofl characteristic for the frequency response of each channel adjacent to the cross-over frequency thereforl I It is also an object of the present invention to provide an improved multiple channel signaltransmission system which operates to provide high and'low-pass filtering withina'predet ermined frequency range and band-pass and band rejection filtering having substantially pure attenuation characteristics without appreciable phase shift.

It is a further object of the invention to provide band-pass and band-rejection filters for relatively low frequency and narrow band ranges, such as from -60 cycles to' any frequency band below cycles without the use of cumbersome and costly circuit elements including large inductances and capacitors.

The invention will, however, bebetter understood from the following description'wh'enconsidered in' connection with the accompanying drawing,iand its scope is pointed out in the appended claims.

In the drawing, which is illustrative of apr'es ent referred embodiment of the invention, Figure 1 isa schematic circuit diagram of a dual- "channel signal-transmission system of the amplifying type, embodying the invention, Figures 2 to 6' inclusive are curves illustrating certain operating characteristics of the 'signal transmission system shown in Figure 1, and Figure 7 is a schematic circuit diagram showing a modification of the output circuit for the system of Figure" 1.

' Referring to'Fig'. 1, the circuit shown is that of an audiofrequen'cy amplifier of the dual-channel type for conveying audio frequency signals in any suitable frequency range, such as from substantially zero to 10,000 cycles, for example, in two bands, such as a low frequency band from substantially zero to 400 cycles, and a high frequency band from 400 cycles to 10,000 cycles, and insuch a manner that the output maybe combined Without phase shift to reproduce signals in'the-original"frequency range. I

Forthis purpose, a two-channel amplifier-is provided. having a low frequency amplifying channel 5 and a high frequency amplifying channe16. Audio frequency signals in the full audio frequency range desired, such as from substantially zero to 10,000 cycles, are received from'any suitable source such as a supply circuit 1-8 which is connected in parallel .to the input circult of each channel. 'Theinput. circuits of channels andfi include an input grid circuit 9 in channel 5 and an input grid circuit ID 'in channel B in each of which is located a coupling network, the constants of which are so chosen that,

, in the high frequency channel, such as channel 5, the input voltage Ede will have substantially zero phase shift from the cross-over frequency down to the lowest frequency in the audio fre- 3 quency range to be transmitted which, in this case, may be substantially zero. 1 this filter network comprises a series resistor I I i In channel 5,

and a'shunt capacitor l2 forming the'series and shunt elements of a filter of the low-pass type, 1 whereas, in channel 6, the series capacitor [3 and a shunt resistor l4 provide the series and shunt elements of a high-pass filter. Choosing a cross-over frequency Fee of 400 cycles, for example, the response characteristic of the two input circuits may be represented as in Fig. 2

by the solid curve l5 and the dotted curve l6, in which the curve l6 represents the input voltage Edc applied to the input grid circuit'9 of channel 5', and the curve represents the input Voltage Eac applied to the input circuit ll] of channel 6.

The low frequency signals Ede are used to modulate a carrier wave Fe in any suitable manner, preferably as shown, by coupling the carrier wave input circuit indicated at H with a sec- 1 ondinput grid circuit [8 of a modulator or mixer tube 19 to which the input circuit 9 is connected, the tube in the present example having an input grid 20 connected to the input circuit 9 and a second input grid 2| connected to the input circuit l8.

The modulated output from the tube is taken through the anode circuit indicated at 22, to

which is coupled a band-pass filter 25 of the bale anced type, which causes the upper and lower 1 side-band phase shifts to cancel. In the present example, the band-pass filter is coupled to the v output circuit 22 through a coupling transformer 26, and the output end of the filter is coupled through asecond coupling transformer 21 with a 3 suitable rectifier device 28, which is preferably of the diode type as shown, connected for full-wave rectification and having an output impedance or resistor 29 from which the rectified output voltage Edc is derived through the leads 30 for connection with any suitable utilization means, not

shown. This voltage is derived from the output voltage. Ebp of the band-pass filter through the j rectifier device, and retains the same zero phase shift characteristic. The attenuation as caused by the filter 25 is as represented by'the curve 3| in Fig. 4.

Referring to Fig. 4, the carrier wave frequency F0 is so chosen that it is above the frequency range of the signal transmission, as indicated in Fig. 3, in which the substantially-zero-to-10,000

cycle range is indicated by the curve 32 with the voltage output characteristic of the modulator indicated by the curve 33. This may be a band 1 of 19,600 to 20,400 cycles about a mean frequency of 20 kcs. in the present example. .The voltage Ebp delivered from the band-pass filter, is indicated by the dotted curve 34 and covers a pass- 1 band equal to the carrier wave frequency less the cut-off frequency, and an upper limit of the carrier wave frequency plus the cut-off frequency. With the cut-off frequency at 400 cycles between the two channels or frequency bands, the width of the pass-band or filter 25 is made substantially 800 cycles. In any case, it is preferably slightly wider than twice the value of the cutoff frequency Fco. A portion of the output voltage Edc from the low frequency channel 5 is taken through a coupling circuit 36 and is applied to the input signal for the high frequency channel 6 substantially out of phase therewith, that is, in phase opposition, and at such amplitude as to exactly cancel that portion of the input signal Eac which falls below the cut-off frequency Fco.

In the present example, the coupling circuit 36 for deriving the low frequency voltage from the low frequency channel is coupled through a network comprising a series capacitor 31 and shunt resistor or impedance 38 to a signal input grid 39 for a combining amplifier tube 40 having a first signal input grid 4| connected with the input circuit I0. With this arrangement, the'input voltage from the circuit l0 and the input voltage from the circuit 30 are combined and the output from the channel is derived from an output circuit 42 through a suitable coupling network comprising a series capacitor 43 and the shunt resistor or impedance 44. This network is included in an output circuit 45 for the high frequency channel. r

The coupling network comprising the capacitor 31 and resistor 38 has the same frequency attenuation characteristic as the input network comprising the resistor l4 and capacitor [3. The manner in which the voltages below the cut-off frequency are applied in opposition to the combining tube or amplifier stage 40 is shown in Fig. 5, the curve 46 representing the input voltage Eac to the channel 6 and the curve 41 representing the input voltage derived from the low fre quency channel output circuit 30 applied in phase opposition to the input voltage Eac to cancel out the signal input to the high frequency channel below the cut-ofi frequency Fco with sharp attenuation at the cut-off frequency because of the characteristic of the signal output from the low frequency channel.

The phase shift and attenuation of the signals through the network l3-l4 is the same as that through the network 31-38 and results in an output voltage E'ac from the channel 6 which has a characteristic represented by the dotted curve 48 of Fig. 5 in connection with a portion 49 of the curve 46 above the cut-off frequency. The over-all output of the two channels E0 may be represented by the combined curves 50 and 5| of' Fig. 6.

By comparing the curve of Fig. 6 with those of Fig. 2, it will be seen that the overlapping of the channels at the cross-over frequency is reduced to a relatively narrow band permitting a more uniform output from the signal transmission system, whether the output from the two channels is combined electrically or in any other manner.

With this arrangement, it will be seen also that signals in any predetermined band of frequencies may b utilized to modulate a carrier wave,

preferably at a higher frequency than the highest operating frequency of the signal transmission system, and that a portion of the modulated wave may be selected by suitable filter means, rectified and applied in phase opposition to the incoming signal in a second amplifying channel to modify the frequency spectrum thereof in a manner to provide high or low passband acceptance or rejection of signals in the second channel without phase shift, whereby pure signal attenuation with sharp cut-off may be obtained.

A low frequency bandpass filter action is provided by two channels like channel 5, one passing signals in a range from zero to 60 cycles, and the other passing signals in a range from zero to 50 cycles. The output of two low frequency channels like channel 5 may be combined as shown in Fig. '7 in which the output transformers 21, rectifirs 28 and rectifier circuits 29-40 are shown connected in opposition to an output circuit 54 from which a signal is derived in a band of 50-60 cycles.

Thus, for low frequency bandpass filter action, two low pass channels are provided, one cutting off at the low frequency end and the other cutting off the high frequency end of the desired passband.

For band rejection, the output from a circuit as in Fig. 7 is applied to the channel 6 as described in connection with the circuit of Fig. 1, resulting in a band rejection in the frequency spectrum of the channel 6.

For variable control of the bandpass, low pass or high pass and band rejection operation, the carrier frequency is preferably high to permit the use of adjustable small capacitors 55 and small variable core inductances 56 in the band pass filter 25 of the low frequency circuit 5 in Fig. 1.

While the invention has been shown and described in its application to a dual channel audio frequency amplifier, it is obvious that it may be applied to other types of multiple channel signal-transmission systems in which it is 'desired to modify the output signal characteristic in a range of signal frequencies with sharp attenuation and without phase shift.

I claim:

1. In a signal transmission system, the combination of means for modulating a carrier wave in a predetermined band of frequencies, said carrier wave being at a higher frequency than the highest operating frequency of the signal transmission system, means including a tunable band pass filter for selecting the carrier wave and side bands of said modulated carrier wave with sharp r;

cut-01f and substantially zero phase shift, means providing a second signal conveying channel, means for applying incoming signals to said channels, means for deriving the modulation component of said modulated signal, and means I for applying said component in phase opposition to a signal in said second channel thereby to modify the frequency spectrum of said signal.

2. In a signal transmission system, the combination of means for modulating a carrier wave in a predetermined band of frequencies, said carrier wave being at a higher frequency than the highest operating frequency of the signal transmission system, means including a tunable band pass filter for selecting the carrier wave and side bands of said modulated carrier wave with sharp cut-off and substantially zero phase shift, means providing a second signal conveying channel, means for applying incoming signals to said channels, means for deriving the modulation component of said modulated signal, means for applying said component in phase opposition to a signal in said second channel thereby to modify the frequency spectrum of said signal, and means for varying the tuning of the band pass filter to vary the modification of said frequency spectrum.

3. In a signal transmission system, the combination of means for modulating a carrier wave in a predetermined band of frequencies, said carrier wave being at a higher frequency than the highest operating frequency of the signal transmission system, means including a tunable band pass filter for selecting the carrier wave and side bands of said modulated carrier wave with sharp cut-off and substantially zero phase shift, means providing a second signal conveying channel, means for applying incoming signals to said channels, said signal applying means including a low pass filter network in the first channel and a high pass filter network in the second channel, means for deriving the modulation component of said modulated signal, means for applying said component in phase opposition to a signal in said second channel thereby to modify the frequency spectrum of said signal, said lastnamed means including a high pass filter network having substantially the same attenuation and phase shift characteristics as the first named high pass filter network.

4. In a signal transmission system for audio frequency signals, means providing a low pass filter for effecting sharp cut-01f at the high frequency end of the pass band of said filter and substantially zero phase shift in signals transmitted therethrough, comprising in combination a signal input filter network for attenuating signals above the high frequency end of said pass band, a carrier wave input circuit, a modulator device coupled to said input circuit and filter network for effecting modulation of a carrier wave in response to received signals through said network, a signal rectifier, a band pass filter providing sharp cut off and cancellation of upper and lower side band phase shift interposed between said modulator and said rectifier, and a signal output circuit for said rectifier for deriving the modulation component of said modulated signal.

5. In a signal transmission system for audio frequency signals, the combination of means providing a low pass filter for effecting sharp cut-off at the high frequency end of the pass band of said filter and substantially zero phase shift in signals transmitted therethrough, comprising in combination a signal input filter network for attenuating signals above the high frequency end of said pass band, a carrier wave input circuit, a modulator device coupled to said input circuit and filter network for effecting modulation of a carrier wave in response to received signals through said network, a signal rectifier a band pass filter providing sharp cutoff and cancellation of upper and lower side band phase shift, interposed between said modulator and said rectifier, a signal output circuit for said rectifier for deriving the modulation component of said modulated signal, andmeans for applying a portion of said derived component in phase -freopiencv carrier wave 1 thereto for effecting last-named channel; 7

7. In a signal transmission system, the .combination of filter meansfor conveying and selecting applied signals with substantially zero phase shift in a relatively wide and in a relatively narrow frequency band, a signal channel for each of said frequency bands coupled to said first-v namedmeans, means coupled to the narrow band frequency channel for applying a relatively h gh frequency carrier wave thereto for effecting modulation of saidcarrier wave in response to applied signals in said relatively narrowiband, a band pass filter in said last-named channel for thecarrier and the side bands of the modulated signal, means for rectifying the output of said band pass filter thereby to derive signals in the narrow frequency band with sharp cut-off and substantially zero phase shift, means for applying a portion of said derived signals to the wide band frequency channel in phase opposition to applied signals in the Wide frequency band to cancel signals in said last-named frequency band in a range determined by the'frequency band of the derived narrow band signals.

Shin a signal transmission system, the combination of filter means for conveying and selecting applied signals with substantially zero phase shift in a relatively wide and in a relatively narrow frequency band, a signal channel for each of said frequency bands coupled to said first-named means, means coupled to the narrow band frequency channel for applying a relatively high frequency carrier wave thereto for effect-' ing modulation of said carrier Wave in response to applied signals in said relatively narrow band, ahand pass filter in said last-named channel for the carrier and the side bands of the modulated signal, means for rectifying the output of said band pass filter thereby to derive signals in the narrow frequency band with sharp cut-ofi and substantially zero phase shift, means for applying a portion of said derived signals to the Wide band frequency channel in phase opposition to applied signals in the wide frequency band, said means including a filter network having substantially the same attenuation and phase shift characteristics as the first-named filter means for said channel for the applied signals.

9. In a signal transmission system, the combination of an input circuit, a low pass filter, carrier Wave modulator means, a band pass filter, a rectifier device and an output circuit in the order named providing. a low frequency signal channel, said low pass filter providing signal attenuation above a predetermined frequency, and said band pass filter being tunable to pass a carrier Wave and the side bands thereof with sharp cut-off and substantially zero phase shift of the side bands, means providing signal channel in said system for conveying signals in a relatively wide band, and means providing a circuit forapplying signals to said second transmission channel from the output circuit of said.v low -frequency. signalchannel in phase opposition to input signals for said second channel, thereby to modifytheoutput of said second gchannel in a predetermined frequency range;

10. In a signal transmissionsystem, the combination of two signal channels comprising two low pass filter means, one cutting off at the low frequency and the other cutting off at the high frequency end of a desired pass band and each of said channels comprising a signal input filter network for attenuating signals above the high frequency end of the pass band, a carrier wave input circuit, a modulator device coupled to said input circuit and filter network for effecting modulation of a carrier Wave in response to received signals through said network, asignal rectifier, aband pass filter providing sharp cutoff and cancellation of upper and lower side band phase shift interposed between said modulator and said rectifier, and a signal output circuit for said rectifier for deriving the modulation component of said modulated signal, means for combining the output from said channels in opposition to produce an output signal having a ban pass characteristic with sharp cut-off.

11. In a signal transmission system for audio frequency signals, the combination of means providing a band pass filter'for effecting sharp cut-oif at each end of the pass band and substantially zero phase shift iii-signals transmitted therethrough, comprising in combination two signal channels each having a signal input filter network for attenuating signals above the high frequency end of the pass band, a carrier wave input circuit, a modulator device coupled to said input circuit and filter network for effecting modulation of a carrier wave in response to received signals through said network, a signal rectifier, a band pass filter providing sharp cutoff and cancellation of upper and lower side band phase shift interposed between said modulator and saidlrectifier, and a signal output circuit for said rectifier for deriving the modulation componentv of said modulated signal, and means for combining the output from said channels in opposition to produce an output signal having a band pass characteristic with sharp cut-off.

12. In a signal transmission system for audio frequency signals, the combination of means providing a band pass filter for effecting sharp cut-off at each end of the pass band and substantially' zero phase shift in signals transmitted therethrough, comprising in combination two signal channels each having a signal input filter network for attenuating signals above the high frequency end of the pass band, a carrier wave input circuit, a modulator device coupled to said input circuit and filter network for effecting modulation of a carrier wave in response to received signals through said network, a signal rectifier, a band pass filter providing sharp cut-off and cancellation of upper and lower side band phase shift interposed between said modulator and said rectifier, a signal output circuit for said rectifier for deriving the modulation component of said modulated signal, means for combining the output from said channels in opposition to produce an output signal having a band pass characteristic with sharp cut-off, and means for applying a portion of the last-named output signal in phase opposition to signals transmitted through said system for cancelling a portion thereof in a frequency range corresponding to said pass band.'

13. A signal-transmission system comprising two channels, means providing filter networks in said channels for passing signals therethrough in complementary frequency bands with minimum phase distortion, means for applying signals to the input circuits of said channels through said network, means in the low frequency channel providing a modulated carrier wave in response to signals applied thereto, a band pass filter connected with said last-named means for deriving therefrom the carrier wave and side bands thereof with sharp cut-off, rectifier means coupled to said band pass filter having an output circuit for the modulation signal, means for deriving a portion of said last-named signal, and means for applying said derived signal in phase opposition to signals in the input circuit of said high frequency channel, said last-named means including a filter network having a predetermined frequency attenuation characteristic.

14. A multiple channel signal-transmission system comprising a low pass channel and a high pass channel, each including an input filter network providing complementary frequency band transmission about a predetermined crossover frequency in a relatively wide predetermined frequency range, means connected with the low pass channel for combining a carrier wave and an input signal from the filter network to effect modulation of said carrier Wave in response to signals, a band pass filter for the carrier and side band frequencies of the modulated signal in said low pass channel following the modulator means, rectifier means coupled to said filter network providing a signal output circuit for said low pass channel, means connected with the high pass channel including a high pass filter network having substantially the same frequency attenuation characteristic as the input filter network for said channel, for applying signals from said output circuit in phase opposition to a signal from said input filter network, thereby to provide sharp low frequency cut-off in said high pass channel.

15. A multiple channel signal-transmission system comprising in combination a signal supply circuit, a low frequency signal channel having an input circuit coupled to said supply circuit and including a low pass filter network, means providing a carrier signal input circuit for said channel, modulator means coupled to said input circuits to receive signals therefrom for modulating the carrier signal with the input signal, a band pass filter in said channel, means for applying the modulated output from the said modulator to said band pass filter, a rectifier coupled to said band pass filter and having an output circuit, a high frequency signal channel having an input circuit coupled to said signal supply circuit and including a high pass filter network, said filter networks being responsive to signals in complementary frequency bands about a predetermined cross-over frequency, and means for applying signals from said output circuit to said high frequency signal circuit in phase opposition to signals in said last-named input circuit, thereby to modify the signal output from the high frequency signal circuit in the pass band of said low pass filter with sharp cut-off at the cross-over frequency in each channel.

16. In a multiple channel signal-transmission system, the combination of a signal input circuit, a signal-amplifying channel coupled thereto including an input circuit having a coupling network for attenuating signals above a predetermined frequency, a second signal-amplifying channel having a second input circuit coupled to said first-named circuit and including a coupling network for attenuating signals below said predetermined frequency, a band pass filter in said first-named circuit providing a pass band of the order of twice said predetermined frequency, means for rectifying the output from said band pass filter to provide low frequency band output for said system with sharp cut-ofi at said predetermined frequency, and means for applying a portion of the rectified output from said filter to the second channel at a predetermined amplitude and in phase opposition to signals therein to provide attenuation of said signals below said predetermined frequency and a high frequency band output from the second channel with sharp cut-off at said predetermined frequency.

17. In a signal transmission system, the combination of a signal conveying channel comprising a carrier wave modulator, a low. pass filter for applying modulation signals to said modulator, a rectifier for the modulated signals having an output circuit, and a tunable band pass filter interposed between said modulator and rectifier for passing a band of signal frequencies having a width of the order of twice the cutoff frequency of the low pass filter, a second signal conveying channel, means providing circuits for applying signals to said second signal conveying channel from the output circuit of the low frequency channel for modifying the signal output thereof in a predetermined frequency range.

18. In a signal transmission system, the combination of a signal conveying channel comprising a carrier wave modulator, a low pass filter for applying modulation signals to said modulator, a rectifier for the modulated signals having an output circuit, and a tunable band pass filter interposed between said modulator and rectifier for passing a band of signal frequencies having a width of the order of twice the cut-off frequency of the low pass filter, a second signal conveying channel, a pair of high pass filter circuits having substantially the same frequency attenuation and phase shift characteristics, means providing circuits for applying signals to said second signal conveying channel from the output circuit of the low frequency channel through one of said last-named filter circuits, and means for applying signals in a predetermined relatively wide frequency range to said second channel through the other of said filter circuits, whereby low pass and high pass filter action is obtained in said signal conveying cir cuits with relatively sharp cut-off at predetermined frequencies.

MAURICE ARTZT. 

